The present invention relates to resistance welding of an aluminum alloy (hereunder referred to merely as aluminum), including resistance welding of aluminum to aluminum, aluminum to steel, aluminum to titanium, and aluminum to other foreign metals.
Joining of aluminum and aluminum can be carried out by arc welding or resistance welding using a large welding current. On the other hand, joining of aluminum and steel, and aluminum and titanium, for example, is usually carried out by mechanical means, because welding does not give a satisfactory bonding strength. However, mechanical joining methods using bolts, screws, and fittings have problems such as less reliability and loosening of joints during use.
Furthermore, since it is becoming more popular to assemble apparatuses, machines, or structures by means of welding, it is desirable that resistance welding of the aluminum, and especially the resistance welding of aluminum plates be practiced in industry using a small welding current.
There is a recent trend to employ increasing amounts of aluminum plates for the manufacture of car bodies in order to save weight and energy. For automotive manufacture it is necessary to develop a practical method of carrying out spot welding of aluminum plates.
A simple and reliable method for joining thick aluminum plates has recently been proposed. This method uses a cladded plate produced by bonding different kinds of metal plates prior to welding as an insert. The opposing sides of a plate and the insert are made of the same metal and are arc welded.
Japanese Unexamined patent application Publication No. 51-72947/1976 discloses an aluminum alloy plate spot welded to a steel plate using as an insert a previously cladded aluminum-steel plate between the aluminum and steel plates to be welded. The purpose of using the insert is to avoid formation of intermetallic compounds of aluminum. However, that application does not disclose any practical method to achieve spot welding of aluminum in an easy way using a small welding current.
Aluminum has high thermal and electrical conductivity compared with steel materials. This means that heat generation in aluminum is rather small compared with steel when the same electric current is supplied to carry out electric resistance welding. In addition, heat, once generated, is easily dissipated through the aluminum. Therefore, highly concentrated heating is required to achieve bonding of aluminum by applying a large electric current in a very short time.
When spot welding of steel plates and spot welding of aluminum plates having the same thickness are carried out for comparative purposes, a welding electric current in an amount three times larger than that for spot welding steel plates must be passed through the aluminum plates to be welded in a period of time only 1/7-1/8 of that for steel plates. Therefore, the spot welding of aluminum to aluminum is carried out with a welding apparatus of a large capacity using an electric current of 20-50 kA in a cycle of 5-10.
In the case of electric resistance welding of aluminum plates to steel plates, when an aluminum clad plate is employed as an insert, an aluminum-to-aluminum bonding interface must be welded by supplying a larger electric current in a much shorter time compared with the current and time required for welding a steel-to-steel interface, sometimes resulting in a variation in bonding strength depending on the location due to formation of uneven nuggets.
Japanese Unexamined patent application Publication No. 4-55066/1992 discloses a method of performing spot welding of an aluminum plate to a steel plate with an aluminum-steel clad plate being inserted between the aluminum and steel plates. In a first stage, an electric current in an amount suitable to effect welding of steel plates is passed through the plates, and then in a second stage a large electric current suitable to effect welding of aluminum plates is used. According to this method, however, two different melting conditions must be created, one for welding steel and the other for welding aluminum. It is rather difficult, therefore, to apply this process to an existing spot welding line from a practical viewpoint. In addition, sometimes excess generation of heat might occur to degrade bonding in the interface between an aluminum plate and the other plate of the clad plate.
Japanese Unexamined patent application Publication No. 4-1288/1992 discloses a method of effecting laser welding of an aluminum plate and a steel plate using a clad plate as an insert. In this method, laser welding is carried out under conditions such that each occurrence of welding takes place locally, i.e., in a very limited area without melting of different types of metals. From a practical viewpoint, it is quite difficult to effect welding in such a limited area even when laser welding is employed. In particular, laser welding is not applicable for welding thin aluminum plates such as outer panels, hood panels, and door panels for use in manufacturing car bodies .